In isolated switching supplies the general principle is that a primary side voltage is switched through a winding on the primary side of a transformer. One or more rectifiers are employed at the secondary side of the transformer in the provision of an output voltage.
Whilst a variety of different topologies are known including Flyback, LLC and Forward converters, the general principle remains.
There is always a general desire to improve the efficiency of switching supplies. The use of a conventional p-n junction diode as a rectifier causes a power loss in the diode arising from a voltage drop across the diode of anywhere between 0.7 V and 1.7 V. This can be improved upon using Schottky diodes which exhibit lower voltage drops (as low as 0.3 volts).
However, further improvements are possible using a synchronous rectifier in which the diode is replaced by an actively controlled switching element such as a MOSFET. The actively controlled switch is switched so as to be on during the same time period that a diode would have inherently switched on. Active switching devices such as MOSFETs have a very low on-resistance meaning a reduction in power loss and a gain in efficiency.
However a known difficulty with using actively controlled switches such as MOSFETs is that the timing for controlling the switch is important to ensure efficient operation. Turning off a switch too early may result in body diode conduction and turning off the switch too late may result in negative current flow. In both of these cases, losses increase. At the same time, it is desirable to control the switch from the primary side since the synchronous nature is generally linked with the operation of one or more switches on the primary side.
To address this issue, various secondary-side “local” control approaches have been used, principally based around current sensing in the rectifier element. Disadvantages associated with these approaches include sensing difficulties in terms of timing and in terms of power loss.
The present application seeks to provide a primary side method of control for a secondary side synchronous rectifier.